Elastin is the chief elastic component of connective and supporting tissues including the walls of major blood vessels. The elastin molecule is synthesized either in fibroblasts or smooth muscle cells. It is exported to the extracellular space where it undergoes fiber formation and chemical modification leading to crosslinks that are either inter- or intramolecular. The crosslinks are important for the property of elasticity. Most of the crosslinks derive from the conversion of certain lysyl residues to corresponding aldehydes by action of a lysyl oxidase. The aldehydes then join with lysyl residues to form aldimine crosslinks that become stable by reduction to a secondary amine crosslink called lysinonorleucine. The aldehydes may also participate in the formation of aldol and desmosine and isodesmosine crosslinks. At the present time no enzyme other than lysyl oxidase is described for these processes. We choose to study the formation of the aldimine and the process by which it is reduced to lysinonorleucine. We shall seek to find if the aldimine and lysinonorleucine formations are enzymatic and determine whether the hydrogen donor to the aldimine is NADH, NADPH, a reduced flavin, a reduced pteridine, ascorbic acid, glutathione or some other yet undescribed donor. Also, we shall explore whether the process occurs in the extracellular milieu or at the surface of cells in such tissues as the aorta. We shall prepare a precursor substrate from chick embryo aortas, and allow an enzyme preparation from aortas to act with suitable cofactors. We shall determine whether tritium from tritiated water becomes incorporated into lysinonorleucine, or whether the hydrogen atom comes from a donor whose transferable hydrogen atom does not exchange with hydrogens of water. We shall study the properties of the putative "aldimine reductase," and follow its developmnt in aortas of chicks, and in aortas of rabbits on hypercholesterolemic diets leading to production of atherosclerosis. The enzyme shall also be sought in collagenous tissues.